Glass Transition Temperature 

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Below tg polymeric material is soft while above is hard? Plz mam justify this question

Below Tg polymeric materials are hard due to no chain mobility whereas above Tg due to chain segment mobility polymeric materials are soft and rubbery.

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Yes. Considering the viscoelastic nature of polymeric material, deformation of a polymer chain is time dependent so any factor which can influence the time e.g., heating or cooling rate will influence the Tg of polymer.

@@PolymerWorld thank you very much!

Softening temperature and glass transition temperature are two different terms. Softening temperature is flow temperature of a polymer. Amorphous polymers are processed at softening temperature as amorphous polymers do not have melting temperature.

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Variation in Tg of same epoxy sample after 30 days could be due to cross-linking over the time or moisture absorption. Also check reproducibility of the sample.

Bulky side group and presence of polar interaction (-OH bond is polar and can lead to inter molecular hydrogen bond) are responsible for higher glass transition temperature of polyvinyl alcohol.

Polystyrene contains bulky side groups (phenyl ring) compared to acetate group in PMA which increases chain rigidity and therefore posses higher Tg than PMA.

Small molecules or oligomers have very low Tg. Due to small chain size these molecules have mobility at very low temperature.

@@PolymerWorld now got it. Thanks a lot Ma'am

Tg is an endothermic process (downward shift in heat flux). In DSC, exothermic transitions occur due to crystallization (during cooling), cold crystallization (during heating), cross-linking and oxidation reactions.

@@PolymerWorld Thank you for your response. I am doing some research work on VBO process using carbon fibre prepregs. I conducted DSC to determine Tg value of the impregnated resin. My plot shows a small upward shoulder just before exothermic peak. I could not find any downward shift in the data. So is it possible for me to get Tg value at the small upward shift just before exothermic peak? if possible, could you please share your official email address? I would like to show you my plots and your help in this regard would be very much appreciated. thank you so much again.

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@@PolymerWorld yasir_mb93@yahoo.com

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Most elastomers show very low Tg. At room temperature elastomers are above Tg.

@@PolymerWorld you mean it have low tg at room temperature

Tg is a temperature at which polymeric materials goes from glassy state to rubbery state. For elastomers, the tg values are typically below 0 degree Celsius so at room temperature which is around 25 degree Celsius, it is above the Tg; therefore it shows high elongation at room temperature.

@@PolymerWorld Ok thanks 🙏🙏❤❤

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Thermosetting polymers undergo curing on heating above glass transition temperature.

@@PolymerWorld Than you for the reply, but what i meant was post curing, lets heat the thermosetting polymer from room temperature to above its Tg, does it become rubbery( viscoelastic)? I had this question because, I read curing is irreversible for thermosetting polymers.

Sorry for late reply somehow didn't see your reply. Yes you read correct curing is a irreversible process. Thermosetting polymers do not show viscoelasticity therefore when heated above Tg they do not become rubbery instead starts to cure and become hard. On further heating instead of getting soft these polymers starts to degrade.

@@PolymerWorld Thank you for the reply. I have a further question, let's say a thermosetting polymer (e.g, Epoxy) is fully cured at say 120 °C (Tg) and cooled down to room temperature. The operation temperature exceeds 120 °C. what will happen?

If the operation temperature is within curing temp. + 50 degree C, post curing process will happen. If the operation temp. is higher than post curing temperature, polymer starts to degrade. Also if the polymer is kept at curing or post curing temperature for prolong period of time, polymer will start to show sign of degradation.

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@@PolymerWorld mam would you please make a series of statistical thermodynamics lecturers.

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